With binoculars, you can see today what no one had seen before Galileo

Jupiter’s “Three Fixed Stars”

By J. Alex Knoll

It was 403 years ago this month, in 1610, that Galileo Galilei trained his telescope at distant Jupiter, and discovered the first four and the largest of its many moons. The first discovery came on January 7, when the Italian scientist wrote of seeing in front of Jupiter “three fixed stars, totally invisible by their smallness.” Lo and behold, when he peered at the objects the next night, he found that they had changed positions, which led to the realization that these were not stars but objects orbiting Jupiter. He named them Io, Europa and Ganymede. A few nights later, he discovered the fourth moon, Callisto.
Despite popular belief, Galileo did not invent the telescope. That distinction goes to the German Hans Lipperhey, who made eyeglasses. Adding curved lenses to either side of a tube, he was able to attain magnification of three times, and he filed for a patent for his invention in September of 1608.
Galileo was, however, the first to use a telescope for astronomy. Hearing of Lipperhey’s invention, he built his own without ever seeing one. Galileo’s telescope could magnify objects 20 times, and with it he studied the moon, tracked the phases of Venus, observed a supernova and discovered those four moons of Jupiter — today called the Galilean moons.
These four moons are by themselves quite bright, with inherent magnitudes between 5 and 6 — at the outer range of being visible to the unaided eye. Yet because of Jupiter’s much greater brightness, typically magnitude –2, and its proximity to its moons, they are all but bleached out of view. With today’s optics, even a modest pair of binoculars is enough to distinguish Io, Europa, Ganymede and Callisto orbiting around Jupiter. Look for the planet shining brilliantly in the east at the onset of evening twilight. Our own moon visits Jupiter on the 21st, when the two are within one degree of one another.